Conventional devices of this type have involved attempts to measure the electrocardiogram signal of a fetus using an abdominal electrocardiogram signal inputted from electrodes positioned on the abdomen of the mother's body. An operation must be performed for separating the feeble signal of the fetus from the abdominal electrocardiogram signal of the mother's body. A variety of methods have been attempted, but a method that is effective regardless of the position and age in weeks of the fetus has not been developed.
In one method currently in actual use, extraction processing is performed using independent component analysis for extracting an electrocardiogram signal primarily on the basis of differences in a probability distribution. However, the electrocardiogram signal extraction magnitude and order cannot be specified, and the method is susceptible to contamination by electromyogram signals and other signals that are close to the probability distribution. Another problem is that the components of a different electrocardiogram of the probability distribution cannot be simultaneously extracted.
The usefulness of fetal electrocardiogram signals is widely recognized, and several extraction algorithms have been proposed. However, few can function on a consistent basis in the various clinical environments; in particular, methods that are effective for the 26th through 36th weeks of pregnancy, in which the SN ratio of the fetal electrocardiogram signal in the abdominal electrocardiogram signal of the mother's body drops precipitously, have not yet been developed.
Fetal electrocardiogram signals inputted from the scalp during delivery have come into primary conventional use due to these problems. This method involves passing directly through the birth canal via the vaginal opening during delivery and attaching a spiral-type unipolar electrocardiogram electrode directly to a portion of the fetus exposed outside the uterus, e.g., to the head or buttocks. Examples of this technique are disclosed in U.S. Pat. No. 6,658,284 to Rosen et al. or in Rosen K G: “Fetal ECG waveform analysis in labour.” Fetal monitoring. Physiology and techniques of antenatal and intrapartum assessment. Ed. SpencerJAD. Castle House Publications. pp. 184-187, 1989.
A method for measuring a fetal electrocardiogram signal obtained from an electrode on the scalp of a fetus is described in U.S. Pat. No. 6,658,284 to Rosen at al. and Rosen K G: “Fetal ECG waveform analysis in labour.” Fetal monitoring. Physiology and techniques of antenatal and intrapartum assessment. Ed. SpencerJAD. Castle House Publications. pp. 184-187, 1989. However, this method is clearly invasive, and the measurement period is limited to the time of delivery. The invasiveness of the method results in increased risk to the fetus of infection and other problems.
In a similar manner, the intrauterine implanted fetus-monitoring device of Japanese Laid-open Patent Application No. 2004-121733 by Horio et al. is also an invasive method in which a hysteroscope in inserted and a microcapsule is directly attached to the fetus. Invasive surgery on the mother's body is necessary in order to attach the microcapsule to the fetus. Application is therefore limited, and this method has the same problems as the example of U.S. Pat. No. 6,658,284.
Examples of fetal electrocardiogram-signal extraction methods hitherto proposed include a direct method according to Ogawa Teruyuki: “RR interval time-series autoregressive analysis of electrocardiograms of normal fetuses and newborns,” The Practice of Time-series Analysis II, Akaike Kouji, Kitagawa Genshirou (editors). Chapter 4, pp. 61-74, Asakura Publishing Co., 1995 by Ogawa et al., in which a template of the electrocardiogram signal of the mother's body is determined and subtracted from the abdominal electrocardiogram signal of the mother's body, and the extraction method of Japanese Translation of PCT International Application No. 2002-538872 by Greenberg et al., which is a mathematically enhanced version of the method of “RR interval time-series autoregressive analysis of electrocardiograms of normal fetuses and newborns” and in which a fetal electrocardiogram signal is extracted using an adaptive-signal processing filter.
In order to obtain the fetal electrocardiogram signal, the aforedescribed methods involve estimating the maternal electrocardiogram and electromyogram signals, which act as noise, and removing these signals from the abdominal electrocardiogram signal of the mother's body, thereby passively extracting the fetal electrocardiogram signal. According to the method disclosed in “RR interval time-series autoregressive analysis of electrocardiograms of normal fetuses and newborns”, the noise components are known to unexpectedly change during fetal movement, uterine contractions, activity by the mother's body, or for other reasons. Conversely, when the noise increases or decreases extremely rapidly, when fat components (vernix caseosa) around the fetus begin to increase as in the 26th through 36th weeks of pregnancy, and the fetal electrocardiogram signal is difficult to detect, or at other times when the SN ratio deteriorates, effective extraction is difficult, and precision is known to conspicuously decrease even in comparison to the precision of extraction using mere independent component analysis (BSS).
On the other hand, well-known methods for extracting a fetal electrocardiogram signal using independent component analysis (BSS) include Taylor M J O, at al.: “Non-invasive fetal electrocardiography in singleton and multiple pregnancies” BJOG, 110, 668-78, 2003 by Taylor et al. and “Fetal electrocardiogram extraction by blind source subspace separation” IEEE Trans. Biomed. Eng., 47, 567-572, 2000 by Lathauwer et al. As described earlier, the electrocardiogram signal extraction magnitude and order cannot be specified in these methods, which are also susceptible to contamination by electromyogram signals and other signals that are close to the probability distribution. Additionally, different components of the probability distribution within the electrocardiogram signal cannot be simultaneously extracted. A signal close to the fetal electrocardiogram signal is sought out from among the several independent components in the analysis results and labeled as the fetal electrocardiogram signal. The type of lead type necessary for the analysis of the electrocardiogram signal cannot be designated, meaning these methods are all passive and therefore entail complications in overcoming the practical problems such as described above.
BSS algorithms have also been proposed. In such instances, a reference signal that is similar to the fetal electrocardiogram signal to be extracted is used, and a signal-source waveform that is strongly correlated to the reference signal is estimated. An example of this method is proposed in “Extraction of event-related signals from multi-channel bioelectrical measurements” IEEE Trans. Biomed. Eng. 47, 583-588, 2000 by Barros at al. In this document, the timing of a periodic function is used as a reference signal, and the periodic function is extracted. However, the number of estimation parameters in this method is enormous. Estimating a 0.5-second phenomenon requires the simultaneous estimation of 100 or more parameters. The algorithm is unstable and the functions that can be estimated are limited. Signal sources that can be used as a reference signal for performing good estimation are also limited. This method is inadequate for actual extraction of a fetal electrocardiogram signal.
Therefore, an effective means for actively and non-invasively retrieving a fetal electrocardiogram signal of a designated lead type from an electrocardiogram electrode positioned on the abdomen of a mother's body has not yet been discovered.